In order to meet increasingly stringent exhaust emission standards, the exhaust emitted from internal combustion engines needs to be treated prior to being emitted. In the context of gasoline engines, catalytic converters have become ubiquitous in the industry to attempt to remove harmful materials from the exhaust. In the near future, diesel engines will be required to have advanced engine emission abatement devices as well. Such abatement devices for diesel engines present different problems than those for gasoline engines. In addition to the formation of complex nitrogen gases, carbon monoxide and raw hydrocarbons, as also occurs in gasoline engines, diesel engine exhaust contains particulate matter or soot, elemental carbon coated with organics comprising residues from unburned fuel and lubricating oils. In addition, the low temperature of diesel exhaust (approximately 150 to 350° C.) compared to the temperature of exhaust emitted from gasoline engines (approximately 900° C.) requires catalysts that can destroy diesel exhaust components under those temperature conditions. Soot, which consists of finely divided carbon and hydrocarbons, is particularly difficult to remove from diesel exhaust at normal operating temperatures.
A device known as a diesel particulate filter (DPF) is one way to remove soot from diesel engine exhaust gas. These filters, made of a porous ceramic or metal substrate, allow the exhaust gases to pass through the filter but trap these small carbon particles. These filters, however, often become clogged with the soot which the engine generates, causing a potentially-harmful backpressure increase in the engine.
So-called active regeneration devices exist that use heat or chemicals (or a combination of both) to remove soot from the filter. Some of these devices operate by spraying raw diesel fuel into the filter chamber and igniting the fuel and soot in situ. This process, along with the presence of oxygen, ignites the soot at a sufficiently high temperature (600° C.) to convert it into either carbon monoxide or carbon dioxide. This process temporarily clears the filter. These devices require a backpressure monitoring apparatus, a fuel injection system and instrumentation to control the monitoring of the filter and the cleaning system.
Passive regeneration devices also exist to remove carbon from the filter, but this removal also occurs at high temperatures (as high as 650° C.). These passive regeneration devices also use large amounts of platinum catalyst, which is an expensive metal.